Optical underwater acoustic sensor

Abstract

An acoustic sensor used in underwater applications. The sensor includes a reflective material adhered to a structure, such as an outer submarine hull or any marine vessel hull. A laser interferometer is placed on the side of the structure with the reflective material. The laser interferometer sends a plurality of laser beams, in sequence or all at one time, to a plurality of points across the retro-reflective material. The laser beams reflect back to the interferometer, which captures the reflected beams using receiving optics. The phase modulation of the reflected laser beams is compared to a reference laser beam within the interferometer to obtain the vibration velocity characteristics of the hull surface structure. Since the reflective material is adhered to the structure, the structure vibration is the same as the vibration of the reflective material. From this vibration, the acoustic pressure associated with the structure may be calculated.

1. An acoustic sensor for detecting an acoustic signature of a structure in an underwater environment, comprising:
 a reflective material adhered to a side of the structure; and 
 at least one laser interferometer, maintaining a reference laser beam, providing laser beams to a plurality of points across the reflective material, having receiving optics to receive a reflection of the laser beams from the plurality of points; 
 a signal processor to measure vibration of the reflective material by comparing a phase modulation of the reference laser beam to phase modulations of the reflection of the laser beams. 
 
   
   
     2. The acoustic sensor of  claim 1 , wherein the reflective material comprises a retro-reflective material. 
   
   
     3. The acoustic sensor of  claim 2 , wherein the laser interferometer produces multiple laser beams. 
   
   
     4. The acoustic sensor of  claim 2 , wherein the interferometer comprises a laser scanner, rotating to provide the laser beams to the plurality of points. 
   
   
     5. The acoustic sensor of  claim 2 , wherein the retro-reflective material is selected from the group of a polymeric material, a reflective paint material, and a material containing glass micro-spheres. 
   
   
     6. The acoustic sensor of  claim 5 , further comprising an acoustically transparent material between the retro-reflective material and the structure. 
   
   
     7. The acoustic sensor of  claim 6 , wherein the acoustically transparent material comprises a polymeric substrate. 
   
   
     8. The acoustic sensor of  claim 5  further comprising a gas on a side of the structure opposite the retro-reflective material, creating a pressure release surface. 
   
   
     9. The acoustic sensor of  claim 1 , further comprising a pressurized gas between the laser interferometer and the reflective material. 
   
   
     10. The acoustic sensor of  claim 9 , wherein the pressurized gas comprises air. 
   
   
     11. A method of detecting acoustic pressure on an underwater structure, comprising the steps of:
 coating a side of the structure with a reflective material; 
 using a laser interferometer, maintaining a reference beam, to provide laser beams to a plurality of points on the reflective material; 
 receiving laser beams reflected back from the reflective material by receiving optics within the laser interferometer; 
 comparing phase modulations between the reference beam and the reflected laser beams; and 
 determining the acoustic pressure from said compared phase modulations. 
 
   
   
     12. The method of  claim 11 , further comprising the step of adhering the side of the structure with an acoustically transparent material prior to the coating step. 
   
   
     13. The method of  claim 12 , further comprising the step of providing a pressurized gas between the laser interferometer and the reflective material.